The invention is concerned with a method for electromagnetic heating by induction heating of a roll, in particular of a calender roll, used in the manufacture of paper or of some other web-formed product, in which method a variable magnetic flux is directed at the mantle of the roll, free of contact, by the intermediate of a magnetic shoe device through air gaps, the said magnetic flux inducing eddy currents in the mantle of the roll, which said eddy currents generate heat owing to the resistance of the roll mantle.
A further subject of the present invention is a paper machine roll device intended for carrying out the method in accordance with the present invention, in particular for the calender of a paper machine, in which said roll device there is a roll mantle arranged as revolving around its central axis, a magnetizing device being arranged in the proximity of the outer face of the roll mantle, which said magnetizing device comprises a number of component cores as well as an electromagnetic coil or coils, by means of which the iron core is magnetized by means of AC electricity.
In respect of the prior art technology related to the invention, reference is made, by way of example, to U.S. Pat. Nos. 4,425,489 and 4,384,514, EP Publication No. 67786, and U.S. patent application Ser. No. 560,394. From U.S. Pat. No. 4,425,489, an electromagnetically heated calender roll is known in which several magnets have been fitted into blocks placed side by side in the axial direction and leaving at least the working area of the outer circumference free, whereat in each block or group of blocks the set value corresponding to the change in the magnetic flux in the mantle of the roll can be varied separately, and whereat, in the roll, at least one temperature measurement-value detector is used, which indicates the measurement value corresponding to the factual-value temperature of the outer face of the roll mantle at different positions placed axially apart from each other, and which said device comprises a control circuit system which changes the set values on the basis of the measurement values and of the predetermined temperature profile for the outer face of the roll mantle.
According to the U.S. patent application Ser. No. 560,394 (applicant Valmet Oy), U.S. Pat. No. 4,614,565 the calender roll is heated inductively by means of eddy currents, and the heating by means of eddy currents is directed on the surface layer of the roll only, made of a ferromagnetic material, and from outside the roll only. According to the said application, an annular thermal insulation layer has been made onto the roll frame, which layer is of a magnetically nonconductive material, and on top of the said layer there is the outer mantle of a ferromagnetic material, whose wall thickness is as little as possible from the point of view of mechanical loads. By means of this arrangement, attempts are made to direct the heating at the heating of the surface layer of the roll mantle only in order to improve the efficiency of heating and to accelerate the adjustment of the temperature profile. The arrangement in accordance with the said patent application is, however, mechanically quite difficult and expensive to accomplish.